Most natural gas plants are designed to condition a feed gas to meet various pipeline sales gas specifications, including Wobbe Index (e.g., 20 MJ/m3), hydrocarbon dew point, and/or water content. In most cases, natural gas plants are used to extract propane plus (C3+) components. However, when the feed gas contains relatively high quantities of ethane (C2), extraction of propane is often not sufficient to produce on-spec product, mostly due to high heating value of the feed gas (mainly caused by excess quantities of ethane).
In general, the main revenue from gas plant operation is generated from sales of the condensate components, which are predominantly propane, butanes, pentanes, and heavier hydrocarbons. Hence, most of the plants are configured to maximize propane recovery. In the past, the ethane content in the feed gas was valued only for its heating content, and there were no significant incentives for ethane recovery. However, with increasing demand from petrochemical facilities to use ethane as a feedstock, ethane can now be sold at a premium price. Considering this market potential, it is thus desirable to have NGL plants for propane recovery with the provision of converting the propane recovery plant to ethane recovery in the future.
Compounding the market demand is the fact that many of today's gas fields contain excessive amount of ethane (13% and higher) that a propane recovery plant would likely fail to meet the Wobbe Index requirement (40 MJ/m3) of the sales gas. Therefore, the natural gas liquids plant must be operated to reject excess ethane in order to meet the sales gas Wobbe Index. However, while many propane recovery plants can be operated on ethane rejection mode, the fractionation of propane becomes less efficient, and propane recovery drops to levels of less than 90% in many cases.
Conceptually, numerous separation processes and configurations are known in the art to fractionate the NGL fractions from natural gas. In a typical gas separation process, a high pressure feed gas stream is cooled by heat exchangers, using propane refrigeration and turbo expansion, and the extent of cooling depends on the hydrocarbon contents and desired levels of recoveries. As the feed gas is cooled under pressure, the hydrocarbon liquids are condensed and separated from the cooled gas. The cooled vapor is expanded and fractionated in distillation columns (e.g., deethanizer or demethanizer) to produce a residue gas containing mainly methane gas and an ethane plus bottom product that is transported by pipeline or other manner to a distant petrochemical facility. Unfortunately, most of the known gas plants process relatively lean gases with an ethane content of less than 10%. While such plants are generally acceptable for feed gas with low ethane content, they are not suitable if the ethane content feed gas is relatively high.
Therefore, known processes may further include an ethane rejection scheme that is needed to meet the Wobbe Index specification, however, often at the expense of desirable levels of propane recovery. For example, Rambo et al. describe in U.S. Pat. No. 5,890,378 a system in which the absorber is refluxed, in which the deethanizer condenser provides reflux streams for both the absorber and the deethanizer while cooling duties are supplied by turbo-expansion and propane refrigeration. Here, the absorber and the deethanizer both operate at essentially the same pressure. Although Rambo's configuration can recover 98% of the C3+ hydrocarbons during propane recovery operation, high ethane recovery (e.g. over 80%) is difficult even with additional reflux streams. Additionally, such configurations are often problematic where the goal is to maintain high propane recovery (e.g. over 95%) when the NGL plant is required to operate under an ethane rejection mode. The rejected ethane will contain a significant amount of propane which typically lowers the overall propane recovery to below 90%. All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
To circumvent at least some of the problems associated with low ethane recoveries, Sorensen describes in U.S. Pat. No. 5,953,935 a plant configuration in which an additional fractionation column and reflux condenser are added to increase ethane recovery using cooling with turbo expansion and Joule Thompson expansion valves for portions of the feed gas. Although Sorensen's configuration may achieve high ethane recoveries, it typically fails to achieve high propane recovery when operated on ethane rejection. Moreover, the C2+ NGL product must be re-fractionated in a deethanizer in most instances to meet LPG vapor pressure specifications, thus increasing the overall energy consumption.
In yet other known configurations, high NGL recoveries were attempted with various improved fractionation and reflux configurations. Typical examples are shown in U.S. Pat. Nos. 4,278,457 and 4,854,955, to Campbell et al., in U.S. Pat. No. 6,244,070 to Elliott et al., and in U.S. Pat. No. 5,890,377 to Foglietta. While such configurations may provide at least some advantages over other known processes, they are generally intended to operate on a single recovery mode, either ethane recovery or propane recovery. Moreover, most of such known configurations require extensive modifications of turbo expanders and pipe routing when the plants are retrofitted from propane recovery to ethane recovery or vice versa. In most cases, the capital and operating cost for the retrofit processes are relatively high and the revenue losses due to facility shutdown for installation are relatively high, thus making an operational change uneconomical.
To circumvent at least some of the problems associated with high ethane recovery while maintaining a high propane recovery, a twin reflux process (described in U.S. Pat. No. 7,051,553 to Mak et al.) employs configurations in which a first column receives two reflux streams: one reflux stream comprising a vapor portion of the NGL and the other reflux stream comprising a lean reflux provided by the overhead of the second distillation column. Similarly, U.S. Pat. App. No. 2010/0206003 to Mak et al. describes an improved natural gas liquid recovery method in which residue gas is integrated to the propane recovery design such that it can be used to reflux the demethanizer during high ethane recovery. Even with these improvements, high ethane recovery (over 90%) is typically not feasible with additional reflux streams.
Thus, although various configurations and methods are known to recover natural gas liquids, all or almost all of them suffer from one or more disadvantages. For example, while some known methods and configurations can be employed for ethane recovery and propane recovery, ethane rejection will typically result in a loss in propane recovery. Moreover, most of the known plants and processes are relatively complex, difficult to operate when changing ethane modes are required, and can typically not produce a pure ethane product as a feedstock to a petrochemical plant.
Therefore, there is still a need to provide methods and configurations for an NGL recovery plant that can recover 95% ethane while maintaining high propane recovery (over 95%) during ethane rejection, and producing a pure ethane product for a petrochemical plant.